PROPOSAL ABSTRACT Most organisms are highly sensitive to fluctuations in the concentration of oxygen (O2), on which they depend to generate adenosine triphosphate (ATP), the cell?s source of energy. O2 deprivation (anoxia) causes a reduction in oxidative phosphorylation and a corresponding decrease in ATP, which is most acutely experienced in organs with high metabolic demand, such as the brain, heart and kidney. However, it has been documented that some organisms can respond to low O2 with a programmed transition into a ?suspended? or hypometabolic state, characterized by a dramatic reduction in ATP consumption via arrest of ATP-dependent processes. This condition has a protective effect on organism viability and is reversible, allowing the organism to resume metabolism and life once O2 is restored. Understanding how to trigger such a hypometabolic state could have dramatic consequences for the prevention of hypoxic/ischemic injury and to promote the viability and storage of organs for transplantation. The zebrafish represents an outstanding model for studying the regulation of hypometabolism. Depending on the stage, embryos exposed to anoxia can arrest development for up to 50 hours and then successfully produce viable adults once O2 is restored. The premise of this proposal is that the identification of key signaling molecules that trigger anoxia-induced arrest in the zebrafish embryo will further our understanding of this process and provide therapeutic targets for protecting patients from hypoxic/ischemic injury. We hypothesize that the metabolite lactate acts as a cellular signal for reduced O2, and triggers metabolic arrest through stabilization and translocation of N-myc Downstream Regulated (NDRG) proteins that block ATP-demanding processes. We will reveal the role of lactate/NDRG1 signaling in adaptation to reduced O2 by pursuing three aims: (1) Test whether lactate/NDRG1 signaling is important for developmental arrest, (2) Investigate the role of lactate in post-translational regulation of NDRG1, (3) Investigate the role of lactate/NDRG1 in arresting the Na+K+ATPase pump.